Exert (6)
'exert' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for strain that I've seen before include "Remove by passing through a filter" , "Overtired" , "Tension - tune" , "Filter - intense effort" , "Sieve; breed" .)
(Other definitions for endeavour that I've seen before include "Try to achieve something" , "The first handle inspector has" , "Determined attempt" , "Undertaking" , "Try - Cook's ship" .)